Ординатура / Офтальмология / Английские материалы / Pediatric Ophthalmology Current Thought and A Practical Guide_Wilson, Saunders, Trivedi_2008

46, 57]. The VIP study demonstrated that the manufacturer’s referral criteria have a sensitivity of over

90% [46], but the low specificity produces substantial over-referrals and a positive predictive value under

10% [44]. The VIP study proposed a second set of referral criteria for the SureSight that are commercially available as a software upgrade for the device and are called the 94% specificity VIP criteria [57]. Their implementation is associated with a higher specificity, a lower sensitivity, a much lower referral rate, and a higher predictive value than the manufacturer’s criteria [44]. Raising the referral criteria for suspected astigmatism from 1.7 diopters (the 94% VIP criteria) to 2.2 diopters decreases the referral rate substantially and increases the predictive value to over 50%.

This modification reduces the referral rate to under 8% and improves predictive value to over 50% [44]; however, these criteria are not currently commercially available.

It is incumbent on any purchaser of the SureSight instrument to specify the desired referral criteria software at the time of purchase. Various referral criteria may be appropriate, depending on the screening situation; specifically the suspected prevalence of the disease of the population, the availability and cost of subspecialty providers for referred patients, and the net direct and indirect cost of over referrals. Areas of the country that have low access to providers and referral subspecialists, and high direct and indirect costs of obtaining care (such as rural Alaska) need to balance a low sensitivity with extremely high specificity and low referral rates, while areas with high population density and adequate primary pediatric eye care capacity may seek a high specificity with less regard for over-referrals (false positives). Further evaluation of autorefraction and adjustment of referral criteria in the future to maximize both sensitivity and specificity will likely continue to increase acceptance of this technology.

6.5.4Required Eye Examinations for Preschool Children

Comprehensive eye examinations have been mandated by state legislatures for at least three individual states in the U.S. Such legislation is usually proposed

as a method for visual evaluation with extensive support from the optometry and optical manufacturing lobby. While in theory such legislation may appear appropriate, there is a lack of legislative mandate for what conditions should be detected, how they should be detected during an examination (i.e., cycloplegia or not) and how identified conditions should be treated. Finally, there is a significant issue with manpower with respect to the vast number of children needing to be evaluated. Recent data regarding the results of this vision screening law in the state of Kentucky have been published [58]. These results demonstrate that spectacles were prescribed for 14% of all children, including 11% of 3-year-olds. This is bothersome because no data demonstrate that 11% of otherwise healthy 3-year-old children require spectacles, and our anecdotal experience from seeing many of these children for second opinions is that cycloplegic refraction is only rarely performed on such children and many spectacle prescriptions are incorrect.

The expense of unnecessary spectacle prescribing for such children has not been fully determined but is substantial [14].

6.6Electrophysiologic Testing

Electrophysiologic detection of decreased acuity has been performed in laboratories for many years using visually evoked response recording. Only recently has this technique been suggested for primary vision screening. The Diopsys device is a portable screening instrument that can be placed in pediatrician offices to screen for amblyopia and other causes of decreased unilateral or bilateral visual acuity. The testing is performed by the clinic staff, and interpreted in real time by integrated software using a proprietary set of referral criteria to determine if a child should be referred. An advantage of this screening instrument to the pediatrician is that the visual evoked response testing is reimbursed by many payers. Disadvantages of this device are a relatively prolonged test time, the need to place scalp electrodes on the child’s head, and the requirement for an attentive and cooperative child. Only one validation study has been published: Simon et al. found that 94% of 122 children aged 6 months to 5 years could be tested, with a sensitivity of 97% and specificity of 81% when this device was evalu-

ated in a pediatric ophthalmology clinic [50]. This low specificity means that most children referred with the current software will have no pathology.

6.7Screening School-aged Children

As children reach elementary school, their visual systems are relatively well developed. Screening in this population presumes that amblyopia has been detected and treated prior to this time and, therefore, the pathology that should be detected in this age group typically is refractive error. Most myopia tends to appear and progress during elementary school. Screening for refractive error in school-age children is typically done both within the school system and the pediatrician offices. The typical test is Snellen visual acuity charts. These charts are time honored, despite the lack of formal field validation. Current standards for elementary school screening suggest referral for acuity worse than 20/30 in either eye [10].At later ages, and in the known myopic child, relying upon subjective symptoms of blurred distance acuity, and traditional acuity screening either in the pediatrician’s office, or by the optometrist or ophthalmologist, is sufficient. It is noteworthy that, in contrast to preschool children, lack of treatment of refractive error, either unilateral or bilateral, will not lead to permanent afferent visual system dysfunction such as amblyopia.

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Contents

7.1.2Examination  . . . . . . . . . . . . .   74

7.2Congenital Ocular MotorApraxia or Saccade

Initiation Failure  . . . . . . . . . . .   75

7.2.1Definition  . . . . . . . . . . . . . .   75

7.2.3Assessment  . . . . . . . . . . . . .   76

7.4.1Assessment  . . . . . . . . . . . . .   80

7.5Delayed Visual Maturation  . . . . . . .   81

7.5.1Definitions  . . . . . . . . . . . . .   81

7.5.3Assessment  . . . . . . . . . . . . .   81

Core Messages

•Children with apparently poor vision can usually be divided into three categories: (1) those with abnormal ocular examination; (2) non-existent retinal findings but abnormal ERG; or (3) those with normal ocular examination.

•Common causes of non-ocular visual impairment include congenital ocular motor apraxia (saccade initiation failure), cortical visual impairment, and delayed visual maturation.

•Understanding the clinical presentation, assessment, and prognosis for

each entity is essential for proper diagnoses and counseling.

7.1Introduction

Inevitably, an ophthalmologist who cares for children will be challenged by an infant with apparently poor vision. In most cases, the causes can be divided into three categories. The first category is comprised of infants in whom the abnormality is apparent after thorough ocular examination. Anterior segment abnormalities may suggest microphthalmus, aniridia, albinism, cataract, or glaucoma. An abnormal fundus

examination could reveal optic nerve abnormalities, abnormal vitreous or retinal pathology suggestive of Leber’s congenital amaurosis, achromatopsia, or congenital stationary night blindness [6, 42]. The second category consists of those infants with subtle or non-existent retinal findings but abnormal ERG. The third category of infants present with the suspicion of blindness and a normal ocular examination. These children are particularly challenging. They are a challenge diagnostically due to frequently associated developmental or neurological deficits [26]. They are a challenge emotionally for parent and physician because the diagnosis to be given varies widely prognostically from a sighted life to one of severe visual impairment.

Caution should be used in labeling a child as blind because the definitions vary between organizations, academic and educational fields, and the public. The World Health Organization (WHO) defines blindness as a corrected visual acuity in the better eye of < 3/60 (20/400) and severe visual impairment as a corrected acuity in the better eye of < 6/60 (20/200) [15, 16].

Both of these acuity levels are far better than complete loss of sight, which can be inferred by the term “blindness.” The WHO epidemiology studies of childhood blindness underscore the discrepancy between causes of visual impairment in regions with higher versus lower socioeconomic status. Westernized countries have significantly higher numbers of nonocular, central nervous system (CNS)-related causes of blindness, a difference which has been attributed both to improved treatment of ocular disease and to perinatal intervention resulting in an increased survival rate of low-birth-weight and premature infants. This change in epidemiology brought cortical visual impairment (CVI), vision loss due to bilateral CNS damage, to the top of the list of childhood causes of blindness in the United States; however, CVI must be distinguished from other forms of visual impairment with normal ocular examinations such as congenital motor apraxia and delayed visual maturation.

This chapter focuses first on history and examination of the child who presents with suspicion of blindness. The more common entities of non-ocular visual impairment are then discussed including congenital ocular motor apraxia, CVI, and delayed visual maturation with attention to particular aspects of clinical presentation and assessment in each condition.

7.1.1History

A thorough evaluation by the pediatrician should and most often does occur prior to ophthalmological examination. Particular attention should be paid to birth history such as prematurity, birth weight, and adverse perinatal events. Discussion of family history may uncover history of consanguinity which is helpful in investigating for evidence of autosomal-recessive retinal diseases.

7.1.2Examination

The absence of “fix and follow” behavior is often the first clue to parent or pediatrician of a visual problem. Infants without this behavior often present after approximately 2 months of age when normally developing children lose the excused “inattention of infancy.” It is an important initial physical examination finding. Fixation and pursuit may be accomplished even in most newborn infants, although its absence may be caused by the infant’s state of alertness. The ideal target is the human face. The physician’s face or, if unsuccessful, the face of the child’s parent can be used during the examination [24].After the human face, bright toys or patterns are useful targets. Once established that the child does not have normal visual responses, it is important to next establish if the child has nystagmus.

Careful examination for nystagmus provides a branching point for common ocular causes of severe visual impairment (Fig. 7.1) [23]. The presence of nystagmus implies anterior pathway disease with the most common anterior causes of severe vision impairment in the U.S. being Leber’s congenital amaurosis, congenital stationary night blindness, bilateral optic nerve atrophy, and achromatopsia [6, 42].

An ERG may be needed to further evaluate retinal disease. Magnetic resonance imaging is useful for structural examination of optic nerve and chiasmal pathology. Only rarely is nystagmus associated with cortical visual impairment. It is important to keep in mind that it is possible for infants to have both posterior and anterior visual pathway damage resulting in a combined clinical picture of nystagmus and CVI. Because a relatively intact posterior visual pathway

Fig. 7.1  Evaluation of the apparently blind infant. (From

[23])

is needed to generate nystagmus, the CNS damage is typically mild in these cases [11].

The importance of refraction cannot be overstated in children with apparently poor vision. High refractive error can be a cause of visual inattention in infants

[44]. It can also lead the clinician to suspect a retinal or systemic abnormality [26]; therefore, careful retinoscopy should be performed after cycloplegia.

In children without nystagmus, the next step is to determine whether they can generate saccadic eye movements. Determination of the infant’s ability to generate saccades may first be established by testing the vestibulo-ocular reflex. This can be done by spinning the infant at arm’s length while facing the physician, and observing slow-phase nystagmus toward the infants’ direction of spin with fast-phase refixation in the opposite direction. The fast phase does not develop until approximately 45 weeks gestational age [5]. Normal full-term infants of 7 days will show both slow and fast phase of nystagmus with rotation; however, premature infants may have deficient or delayed responses [5, 10]. If the infant fails to develop a fast phase, this is evidence that they cannot generate saccades and there can be no conclusions drawn from the “fix and follow” test. These children may have congenital ocular motor apraxia or forms of saccadic palsies which may be seen in children with CNS damage [24]. It is also important to consider that the vestibulo-ocular reflex can occur occasionally in the absence of vision, i.e., it is a brain-stem reflex.

If the infant without nystagmus demonstrates the ability to generate saccades and has a known history of CNS damage, the diagnosis of cortical visual impairment should be considered. Evidence by imaging of damage to the occipital cortex or radiations confirms the suspicion. The diagnosis of CVI applies only to those infants with damage limited to the visual pathways, rather than more generalized neurological disease.

Finally, in the infant with normal ocular examination, no nystagmus, and without evidence of neurological injury, application of the term delayed visual maturation is appropriate. In these children, vision improves quickly, usually within 3−5 months of age.

7.2Congenital Ocular Motor Apraxia or Saccade Initiation Failure

7.2.1Definition

Congenital ocular motor apraxia (COMA) is a condition described first by Cogan as a deficit in the voluntary initiation of horizontal saccades [8]. Children with COMA show an abnormality in both the initiation and amplitude of voluntary and optically induced horizontal eye movements, characterized by a failure of quick-phase nystagmus. Vertical eye movements typically remain normal, but vertical saccade initiation can occur and suggests more serious CNS disease. The term “intermittent saccadic failure” was later suggested by Harris et al. [22] due to the fact that a “true” apraxia consists of abnormal voluntary saccades but normal reflexive movements upon testing.

7.2.2Clinical Presentation

Children older than 3 months with COMA exhibit a characteristic horizontal head thrust associated with attempted ocular refixation. When they attempt to look at an object, they first turn their head toward and beyond the object of interest while their eyes rotate in the opposite direction appearing “left behind.” The head thrust forces the eyes to initially deviate even further away from the target, and the child must turn

the head past the object (“overshoot”) in order to engage the object. When their eyes engage the object of interest, they then unwind their head counter to the direction of the thrust allowing the eyes to remain fixated on the object. A prominent blink may accompany the saccadic movement. This “synkinetic blinking” represents an adaptation used primarily in older children to aid in the initiation of saccades (Fig. 7.2)

[34]. The head thrust is essentially the child’s use of the vestibulo-ocular (doll’s head) reflex to generate horizontal movement. The head thrust diminishes during the first decade as ability to initiate saccades improves or as the child adapts to the saccadic deficiency [35, 45] Nevertheless, virtually all children with COMA experience difficulty reading.

The diagnosis of COMA may be more difficult in infants younger than 3 months of age [14] prior to development of head control and ability to execute the head thrust. These children with COMA seem simply visually inattentive. They often present early in life with a diagnosis, from the referring physician, of poor vision or failure to fix and follow because of their lack of normal ocular refixation movements [34].

7.2.3Assessment

Direct observation of the characteristic head and eye movements in children with COMA is often enough to make the diagnosis. Testing of vestibular-ocular reflexes and OKN drum can be helpful in confirming and differentiating the diagnosis from other entities with head thrust such as gaze palsy, slow saccades, visual field defects, or poor eccentric gaze holding [22]. The OKN drum should demonstrate normal vertical saccades and pursuit. The affected child’s eyes appear “locked up” with OKN spun horizontally.

This occurs as the eyes are driven toward the direction of movement but do not show normal fast-phase saccade recovery. It is important to note that testing with a handheld OKN in the clinic may give the false impression of deficient saccades because the eyes are not driven to the limit of gaze. Full-field OKN will demonstrate the characteristic deviation.

The vestibulo-ocular reflex, together with the opto-kinetic system, is responsible for holding images steady on the retina. This reflex is driven by movement of endolymph fluid in the semicircular

canals and is not visually mediated; therefore, it can be elicited in the dark [5]. In children with COMA, testing of VOR by rotation of the infant will cause the eyes to deviate and remain in the furthest extent of the slow phase (toward the direction of the infant’s rotation).

Vision, ERG, and VEP should be normal in children with COMA. Any abnormality on these tests should prompt further investigation for associated neurological disease [5].

ten attributed to the underlying abnormalities. There is evidence that even children with isolated COMA may still have associated developmental delay in motor, speech, or behavior [32, 34].

Older, school-age children tend to show less clinical signs of the condition. It is not known whether the condition itself improves or if the improvement is due primarily to the child’s adaptive capabilities. There is a diminished use of the head thrust in older children, as well as use of an exaggerated blink prior to fixation on a target (synkinetic blinking) to replace or augment head movement [5, 22].

7.2.4Etiology

COMA is described as a sporadic disorder, although familial cases have been documented [9, 35, 45] including a suggested autosomal-dominant transmission pattern in one pedigree [35]. The pathophysiology of COMA is unclear. It may occur in an idiopathic form or in association with structural or systemic neurological disease. The most commonly reported structural abnormalities include agenesis of the corpus callosum and cerebellar hypoplasia [5, 8, 14, 22, 34]. Structural abnormalities in the cerebrum, cerebellar vermis, brain stem, mid-brain, and basal ganglia, as well as perinatal insults such as hypoxia, cerebral palsy, and hydrocephalus, have also been associated [8, 22, 14, 38]. The finding of vertical saccade failure suggests these more ominous conditions.

In addition to a congenital etiology, ocular motor apraxia can occasionally be acquired and associated with (1) neurodegenerative disorders such as ataxia−telangiectasia, Gaucher’s disease, and linear sebaceous nevus syndrome; or (2) acquired disease such as posterior fossa tumors, ischemia, and herpes encephalitis, among others. These children initially exhibit normal development, followed by unexplained neurological decline.

7.2.5Prognosis

As stated previously, congenital ocular motor apraxia may be idiopathic or associated with underlying structural or systemic neurological disease. In children with underlying disease, the deficits are most of-

7.3Cortical Visual Impairment

7.3.1Introduction

Cortical visual impairment (CVI) is defined as bilateral loss of central vision due to damage to the CNS. Anterior visual pathways (globe, optic nerve, chiasm) are spared, while damage to posterior visual pathways (lateral geniculate body, optic radiations, primary visual cortex, visual association areas) produces variable visual deficits. Cortical visual impairment is the leading cause of bilateral vision impairment in children in Western countries This is believed to be due to higher survival rates of children with perinatal hypoxia and ischemia as well as improved treatment of previously lethal diseases [17, 20].

A discussion of cortical visual impairment cannot occur without clarification of terminology. Although the terms cortical or cerebral “blindness” have been used to describe patients with visual impairment related to CNS injury, it should not be used as a diagnostic term for children. Blindness implies total loss of vision and is not typical of children with CVI. In contrast to adults with acquired CNS injury, children with CVI routinely preserve residual vision. Considerable debate has also surrounded the use of cortical versus cerebral to describe the site of impairment. It can be argued that cerebral impairment is a more accurate term since the CNS damage can occur anterior to the visual cortex. At this time, “cortical visual impairment” is our preferred description of the condition due to the emphasis on the striate cortex as the ultimately disrupted endpoint of the posterior pathway.

7.3.2Etiologies

Perinatalhypoxia/ischemiaisthemostcommoncause of CVI [4, 17, 20]. The pattern of CNS damage differs for premature infants versus full-term infants and is best explained by examination of the difference in blood supply in the developing brain.

In infants prior to 34 weeks gestation, the watershed zones between the major cerebral arteries

(posterior, middle, anterior cerebral arteries) are also supplied by meningeal anastomoses [28]. This protects the area between the three major arteries, called the para-sagittal region, from infarctions. However, another watershed zone exists transiently in the premature infant in the periventricular area (choroids plexus). It is supplied by ventricular branches of deep penetrating arteries. Ischemic damage occurs to this periventricular white matter adjacent to the lateral ventricles resulting in involvement of optic radiations (Fig. 7.3). Periventricular leukomalacia

(greek origin. leuko “white” + malakia “softness”) is the most common form of hypoxic damage in premature infants. Risk factors in the premature infant include first-trimester hemorrhage, maternal urinary tract infections, neonatal acidosis at birth, meco- nium-stained amniotic fluid, and premature rupture of membranes [28]. Full-term infants manifest the para-sagittal watershed zones between the three major systems as described above, but they lack the additional meningeal anastomoses protective in the premature infant (Fig. 7.4). The areas between the parieto-occipital lobe as well as the body of the caudate nucleus are most vulnerable to both hypoxia and hypotension as they are “triple” watershed zones [4].

Causes for cerebral dysfunction after hypoxia are largely unknown. Proposed pathophysiological mechanisms include glutamate toxicity, free radical injury and macrophage-mediated cytokine damage, interruption in protein synthesis in neural or glial

Fig. 7.3  Chronic, severe periventricular white matter injury. T2-weighted axial magnetic resonance image of the brain at the level of the lateral ventricles. Note marked white matter loss in the posterior periventricular white matter with lack of space between ventricles and overlying cortex. Ventricles are dilated posteriorly with an irregular and scalloped border. (Adapted from [46])

Fig. 7.4  Patterns of brain injury after hypoxic/ischemic insult. The premature infant brain (left) has a periventricular area, supplied by deep penetrating arteries, which is vulnerable to ischemic damage. In the term infant (right), the parasagittal “watershed zone” between the three major arteries is the most susceptible to injury. (Adapted from [47])